1. Field of the Invention
The present invention relates to a wet friction material used for a clutch, a brake or the like in a wet friction engagement device.
2. Description of the Related Art
In order to understand the present invention, an example of a wet friction engagement device is illustrated in FIG. 3. In FIG. 3, a driving plate 2 engaged with a spline portion 51 of a hub 5 fitted to an input shaft 6 comes into contact with a driven plate 1 engaged with a spline portion 41 of a retainer 4, so that torque is transmitted. The reference numeral 3 represents a pressure plate; and 7, a piston for applying pressure.
In view of current energy and environmental problems and the like, such a friction engagement device is required to be small in size and light in weight, small in operating shock, and high in torque capacity. At the same time, the friction engagement device is required to cope with high energy of a car engine with high-velocity rotation and high output of the same. The requirement is extremely high.
A friction material to be stuck onto the surface of a driving plate and a driven plate was formed from natural pulp or synthetic organic fibers (aramide fibers, etc.) as a base material.
That is, practical examples of the compounding ratio in a conventional friction material are as follows. In a first example (referred to as "Example J-1"), the friction material is composed of natural pulp by 35%, synthetic organic fibers by 5%, a filler by 30%, and phenol resin by 30%. In a second example (referred to as "Example J-2"), the friction material is composed of natural pulp by 20%, synthetic organic fibers by 20%, a filler by 30%, and phenol resin by 30%.
Synthetic organic fibers such as aramide fibers are expensive while they are superior in heat resistance. The compounding ratio of the synthetic organic fibers tends to increase in order to improve the heat resistance, so that the price has risen correspondingly. Accordingly, it was considered to use carbon fibers to reduce the price. Carbon fibers are however rigid and do not intertwine with each other or with any other fibers.
Accordingly, there was a defect that the strength of friction material was insufficient, so that it could not be made up into a product. Further, it was impossible to avoid various problems that the heat radiation and flexibility were not as good as expected, and the like.